Determination of bisphenol A in commercial cat food marketed in the Czech Republic

Abstract

Objectives

Bisphenol A (BPA) is one of the most widely used synthetic compounds on the planet. It is used in the synthesis of polycarbonate plastics, epoxy resins and other polymer materials. Owing to its excellent chemical and physical properties, it is used to produce food and beverage containers or the linings for metal products. BPA has been mentioned as a possible cause of feline hyperthyroidism. Cat food is considered one of the main sources of BPA intake. The purpose of this study was to evaluate BPA concentration in various types of commercial cat food available in the Czech Republic.

Methods

In total, 172 samples prepared from 86 different types of commercial cat food were assessed. The concentration of BPA was measured using liquid chromatography–tandem mass spectrometry.

Results

Measurable concentration of BPA was found in all samples (range 0.065–131 ng/g), with the highest concentration (mean ± SD) of BPA in canned food (24.6 ± 34.8 ng/g). When comparing BPA concentration in food trays (1.58 ± 0.974 ng/g), pouches (0.591 ± 0.592 ng/g) and dry food (1.18 ± 0.518 ng/g), concentrations of BPA in food trays and dry food were significantly higher (P <0.01) compared with pouches. Comparing BPA concentrations in canned food of different manufacturers, statistically significant differences were found as well.

Conclusions and relevance

The highest concentrations of BPA were found in cans. Thus, cans represent the highest possibility of exposure to BPA in comparison with other types of commercial feline food.

Keywords

Hyperthyroidism canned food food tray pouch

Introduction

Bisphenol A (BPA) is a synthetic organic compound widely used in industrial production, mainly in the synthesis of polycarbonate plastics and epoxy resins. Owing to their excellent chemical and physical properties, including good strength and hardness, thermal stability and resistance to acids and oils, they are extensively used to produce food and beverage containers. The epoxy resins are mainly used to produce inner linings for metal products, especially for food cans.¹

Due to its mass production and widespread applications, the presence of BPA is ubiquitous in the environment. Various manifestations of toxicity were reported in humans, domestic and laboratory animals, and animal models. BPA is considered as a substance with endocrine disrupting effects.¹ The food from BPA-containing cans is considered a main exposure source and the digestive tract is the largest site of absorption.²

In humans, BPA may affect the levels of male and female sexual hormones,^3,4 fertility and sperm quality,^5–7 may cause developmental disorders,^8,9 metabolic disorders – obesity,^10,11 type-2 diabetes^12,13 and may affect liver and kidney function.^14–16 In addition, increased incidence of various types of tumours is associated with BPA (breast, prostate gland, ovaries, lungs).^17–19

In cats, feeding canned food was identified as a risk factor for hyperthyroidism.^20–23 This can be explained, at least in part, by the fact that the food is contaminated by BPA from the internal lining of metal cans.^24–27 Because of the structural similarity of BPA and thyroid hormones, deleterious effects of BPA on thyroid function have been postulated.²⁸ The results of in vitro study suggested that BPA inhibits triiodothyronine binding to the thyroid receptor.²⁹ In laboratory rats, BPA was able to increase thyroxine serum concentration and increase the production of reactive oxygen species in thyrocytes, which may predispose to thyroid disease.^30,31

The key role of the internal coating of cans as a main source of BPA contamination of canned food is well known. However, complex assessment and comparison of different types of feline food in relation to BPA content and possible health risks is still missing. The purpose of this study was to assess BPA content in different types of feline food available in the Czech Republic originating from several manufacturers.

Materials and methods

Samples

A total of 172 commercial cat foods that are commonly available in pet shops in the Czech Republic were purchased from four pet food companies. The samples used in our study are represented by four types of cat foods: pouches (n = 62); food trays (n = 30); cans (n = 64) and dry food (n = 16). The characteristics of the cat foods with declared constituents are summarised in Supplementary Table 1 in the supplementary material. Each pet food sample was purchased in two replicates at the same time from the same site. The unopened feline food was stored at room temperature. All feline foods were analysed before ‘best-by dates’. After the cans were opened, the total contents of each can were immediately and thoroughly homogenised in a glass beaker with a wooden spatula to avoid possible bisphenol contamination and then used for analysis. The internal surface areas of pouches, food trays and cans were calculated. Dry food (10 g) was homogenised using a mortar and pestle.

Materials and reagents

Standards of BPA and isotopically labelled BPA-D₁₆ (internal standard), sodium bicarbonate, sodium hydroxide, dansyl chloride and formic acid were purchased from Sigma-Aldrich. Acetonitrile and methanol were purchased from Chromservis (Czech Republic) and were LC/MS purity (⩾99.9%). Nylon syringe filters were purchased from Millipore.

Determination of BPA in cat food

The determination of BPA in cat food was conducted using liquid chromatography–atmospheric pressure chemical ionization–tandem mass spectrometry (LC/MS). A sample of cat food (0.5 g) was spiked in a glass test tube with internal standard solution (5.0 μl) at a target level of 1.0 ng/g and vortexed with acetonitrile (2 ml) by a glass rod for 10 mins and then extracted in an ultrasonic bath (Bandelin Electronic GmbH & Co) for 15 mins. After extraction, samples were centrifuged at 800 g for 15 mins at 20°C and the supernatant was collected in a glass test tube. Extraction was repeated, and both supernatants were combined and filtered through a paper filter (15 μm, 15s KA-1 filter paper; Pulp & Paper Mills Pernštejn). Filtered supernatant (0.5 ml) was used for derivatisation with dansyl chloride.³² Sample solution was evaporated until dry under a gentle nitrogen stream (at 55°C); a volume of 250 μl of sodium bicarbonate buffer (100 mM; pH 10.5) and 250 μl of dansyl chloride solution in acetone (1 g/l) were added into dry residues in the glass test tube, gently vortexed and incubated at 60°C for 5 mins. Subsequently, the mixture was evaporated until dry under a gentle nitrogen stream (at 55°C); dry residues were reconstituted in 500 μl of methanol, filtered through a 0.2 mm nylon filter (Millipore) and used for LC/MS analysis. Conditions of LC/MS analysis based on ultra high-performance liquid chromatography connected to a triple quadrupole mass spectrometer with atmospheric pressure chemical ionisation (Thermo) are described in our previous article.³³

For our quality assurance/quality control program, the instrument was calibrated daily with multi-level calibration curves. Procedural blank and solvent blank were analysed for every set of five samples. To evaluate method process efficiency (PE), extraction recovery (RE) and matrix effect (ME), 10 samples and 10 sample extracts were spiked with standard solution at three target levels (0.06; 6.0 and 60 ng/g). The matrix effect was calculated as ME (%) = (B/A – 1) × 100, where A is the peak area of the standard solution and B is the peak area of the post-extraction standard addition. The extraction recovery was calculated as RE (%) = C/B × 100, where C is the peak area of the pre-extraction standard addition and the process efficiency was calculated as PE (%) = C/A × 100. In order to determine inter-day precision and accuracy, samples were spiked (pre-extraction standard addition) at three target levels (0.06, 6.0 and 60 ng/g) and analysed at intervals over a 2-week period (n = 10). The lower limit of quantification (LLOQ) was defined as the lowest concentration level of calibration curve (0.06 ng/g) with precision and accuracy of less than 20% and the corresponding signal/noise ratio greater than 5. Results of validation are included in Supplementary Table 2 in the supplementary material and sample chromatograms are included as Supplementary Figure 1 in the supplementary material.

To avoid contamination of samples and solvents, all operations were performed without plastic, using glassware with one exception. The only plastic used in the described procedure was the nylon filter and plastic syringe. However, the nylon filter and plastic syringe were used for filtration after derivatisation. Thus, they did not contribute any contamination of dansylated BPA to the procedural blank.

Statistical analysis

Statistical analysis was performed using Statistica 8.0 for Windows (StatSoft). Concentrations of BPA in cat food were log₁₀ transformed to approach a normal distribution. An analysis of variance (ANOVA) was used to calculate the effect of type of packaging and effect of manufacturer. When a significant effect was found, a conservative Tukey’s test was conducted as a post hoc test to determine BPA differences between individual groups. An unpaired Student’s t-test was used to evaluate effects of manufacturer in cases where two data sets were available. The relationship between BPA content and declared constituents as well as internal surface was assessed by Spearman correlation. Significance was set at P<0.05. Data of BPA concentrations in text are presented as means ± standard deviation.

Results

Measurable concentration of BPA was found in all samples (range 0.065–131 ng/g). BPA concentrations are shown in Supplementary Table 1 in the supplementary material.

In the case of BPA concentration expressed on a wet weight basis, ANOVA revealed an effect of packaging type (P <0.01). When using the post hoc Tukey’s test, concentrations of BPA were significantly higher (P<0.01) in cans (24.6 ± 34.8 ng/g) compared with all other types of packaging (Figure 1), and concentrations of BPA in food trays (1.58 ± 0.974 ng/g) and dry food (1.18 ± 0.518 ng/g) were significantly higher (P <0.01) compared with pouches (0.591 ± 0.592 ng/g).

Figure 1

Concentrations of bisphenol A (BPA) in (a) all samples and (b) in all samples expressed on dry weight basis food trays (n = 30); pouches (n = 62); cans (n = 64) and dry food (n = 16). Boxes show the median and the upper and lower quartiles. Whiskers show the range without outliers (OLs). In the case of cans, there were six OLs in a range from 120 to 131 ng/g and in a range from 600 to 655 ng/g expressed on a dry weight basis, respectively. OLs were defined as OL = UQ + 2·(UQ – LQ), where UQ is the upper quartile and LQ is the lower quartile. Different letters indicate significant differences (P <0.01)

In the case of BPA concentration expressed on a dry weight basis, ANOVA revealed an effect of packaging type (P <0.01). When using the post hoc Tukey’s test, concentrations of BPA were significantly higher (P <0.01) in cans (122 ± 176 ng/g) compared with all other types of packaging (Figure 1). Concentrations of BPA in food trays (7.48 ± 4.41 ng/g) were significantly higher (P <0.01) compared with dry food (1.29 ± 0.569 ng/g) and pouches (3.19 ± 3.13 ng/g).

When evaluating differences between manufacturers, ANOVA revealed an effect of manufacturer (P <0.01) on concentration of BPA in cans (Figure 2). When using the post hoc Tukey’s test, concentrations of BPA were significantly higher (P <0.01) in cans from manufacturers 1 and 3 compared with manufacturers 2 and 4, and concentrations of BPA were significantly higher (P <0.01) in cans from manufacturer 4 compared with manufacturer 2. ANOVA did not reveal any effect of manufacturer (P >0.05) on the concentration of BPA in pouches (Figure 2). An unpaired Student’s t-test did not reveal (P >0.05) differences of BPA in food trays between manufacturers 3 and 4 (Figure 2). No significant correlations (P >0.05) between BPA content and declared constituents as well as internal surface area were found.

Figure 2

(a) Concentrations of bisphenol A (BPA) in cans based on manufacturer: manufacturer 1 (n = 14); manufacturer 2 (n = 14); manufacturer 3 (n = 24); and manufacturer 4 (n = 12). (b) Concentrations of BPA in pouches based on manufacturer: manufacturer 1 (n = 16); manufacturer 2 (n = 20); manufacturer 3 (n = 14); and manufacturer 4 (n = 12). (c) Concentrations of BPA in food trays based on manufacturer: manufacturer 3 (n = 16); and manufacturer 4 (n = 14). Boxes show the median and the upper and lower quartiles. Whiskers show the range. Outliers are marked with circles and defined as OL = UQ + 2·(UQ – LQ), where UQ is the upper quartile and LQ is the lower quartile. Different letters indicate significant differences (P <0.01)

Discussion

The main goal of this study was to compare basic types of packaging for commercial cat food in relation to BPA content. Our results from samples purchased in the Czech Republic showed that cat food from lined metal cans contains the highest amount of BPA in comparison with all other types of commercial cat food. This finding confirms that linings of the interiors of metal cans are the main source of BPA contamination of canned food. Epoxy resins are predominantly used as an internal coating in cans to prevent contact between the food and the metal wall.³⁴ According to the studies that tested leaching of BPA from internal coatings, BPA can be released in relatively high concentrations by short-term leaching.^26,27

The concentration of BPA in the cat food contents from cans ranged from 0.430 to 131 ng/g. This range is comparable with concentrations in canned pet food reported in the majority of previous studies. Kang and Kondo reported that BPA concentrations ranged from 13 to 136 ng/g in canned cat food and from 11 to 206 ng/g in canned dog food from USA, Australia, Canada, Japan, Korea, Philippines and Thailand.²⁷ Koestel et al reported BPA concentrations of 11.8 ± 4.3 ng/g and 18.0 ± 3.6 ng/g respectively, in canned dog food from two pet companies purchased in Missouri, USA.²⁶ Cerkvenik-Flajs et al reported that BPA concentrations in canned dog food (retailed on the Slovenian market) ranged from 14.0 to 208.1 ng/g, with the exception of one sample under the detection limit (5 ng/g).³⁵ On the other hand, Schecter et al reported a very low content of BPA in canned cat and dog food (from Texas, USA) in a range from 0.23 to 0.32 ng/g and with several results under the detection limit (0.20 ng/g).³⁶ The relatively wide range of BPA concentrations in cans (four orders of magnitude) in our study is due to significant differences between pet food companies. These results may indicate significant differences in composition of internal coatings.

The significantly lower BPA concentrations in pouches, food trays and dry food in comparison with cans can be attributed to the lack of internal coating. Food trays are manufactured from aluminium without internal coating and the literature is deficient, so comparison is not possible. Pouches are manufactured from plastic and the very low content of BPA indicates that the plastic of pouches is probably BPA free. Schecter et al reported BPA concentrations in cat food from plastic packages under the detection limit (0.20 ng/g).³⁶ Although BPA concentrations in pouches, food trays and dry food were significantly lower in comparison with cans, measurable concentrations of BPA were detected in all evaluated samples, not only in food packaged in metal cans.

Regarding the reported presence of BPA even in non-canned food, the contamination of the primary raw material used for production of pet food may be a possible explanation.^37–40 The pet food might also be contaminated during manufacturing and pH may also play a significant role in BPA migration from the coating to canned food. Schecter et al reported that BPA concentrations were greater for foods with pH 5 than for lower (more acidic) or higher (more alkaline) levels.³⁶ Stojanovic et al reported that migration of BPA from the epoxy-phenolic coating to food is higher when the food is more acidic.⁴¹ In contrast, Biedermann-Brem and Grob reported substantially increased liberation of BPA when the pH is increased.⁴² The results of the available literature are controversial; the actual impact of pH on BPA release is unknown and further studies are needed.

All the cat foods were stored at room temperature and were analysed before the expiry date. The actual age of packed cat food was not recorded. According to the previous reports, there are no significant effects of storage period and BPA concentrations in older cans were not higher.^43–45 Munguía-López et al concluded that the highest migration of BPA occurs during manufacturing when high temperature is used for prolonged time.⁴⁶ Further migration of BPA into the can content seems to be minimal.²⁴ When inner and outer layers of canned food were compared, there were no significant differences in BPA concentration.⁴⁷

The explanation for outliers (Figures 1 and 2) is unclear and there is a lack of literature data. Nevertheless, since the concentrations of outliers are higher than other values, there is a possibility of higher contamination of the ingredients used or additional contamination during the manufacturing process. We can exclude contamination during analysis because the procedural blank and solvent blank were analysed simultaneously with samples as described in Materials and methods.

The comparison of BPA concentrations expressed on a dry weight basis did not change the main findings. Concentrations of BPA were still significantly higher in cans compared with pouches, food trays and dry food. However, as expected, calculation on a dry weight basis was advantageous for dry food, with the significantly lowest content of BPA expressed on dry weight.

In comparison with pet food, the scientific literature evaluating BPA concentrations in canned food for humans is much more abundant and reported results show huge differences according to the type of food and selected methodology. The concentrations of BPA in human canned food range from less than 0.1 ng/g to almost 1000 ng/g in canned meat, seafood, milk, fruits and vegetables. For more information about BPA in human canned food, see reviews by Repossi et al and Russo et al.^48,49

The highest concentration of BPA measured in this study was 131 ng/g. The manufacturer recommends approximately 300 g of this food for cat with a body weight of 4 kg. When a cat is fed with this diet only, it would exceed 39.3 µg of BPA per day, which is equivalent to 9.8 µg/kg/day. This amount is more than twice the tolerable daily intake (TDI) for humans set by the European Food Safety Authority (EFSA), which is 4 µg/kg/day.⁵⁰

Since canned food is considered a significant source of BPA, many pet food companies declare their cans as BPA free. However, Koestel et al, in their study in the USA, did not find a significant difference in BPA content between canned dog food declared and believed to be BPA free and canned dog food without BPA-free declaration.²⁶ Although this fact cannot be generalised, BPA-free declaration by food companies regarding canned food may be an inadequate or unreliable assurance if we want to lower the dietary risk caused by BPA.

Human and animal studies show that exposure to diets and beverages containing BPA leads to increase of internal concentrations of BPA.^26,51–56 Various effects of BPA on health are reported in humans and laboratory animals. It is generally believed that BPA has shown endocrine-disrupting effects on various biological receptors. It consequently results in health hazards for many systems, such as reproductive, nervous, and immune systems, as well as for metabolic function, growth and development of offspring (reviewed by Ma et al).¹ Nevertheless, the information from pets is lacking. The most relevant study is from Koestel et al (2017), where dogs fed with canned food showed a significant increase in circulating BPA concentrations.26 This short-term dietary exposure to BPA resulted in fecal microbiome alterations. Potential BPA-induced changes in the gut microbiome may, in turn, lead to alterations in other metabolic pathways. Thus, similar studies evaluating direct effects of feeding canned food with BPA in cats are needed.²⁶ There are only two very recent experimental studies in cats and one report evaluating BPA concentration in feline serum and its association with clinicopathological findings,^33,57,58

Despite the fact that BPA has been mentioned as a possible factor contributing to feline hyperthyroidism, little is known about its metabolism in cats and impact on thyroid function. Kovarikova et al did not find a significant association between BPA and serum concentration of thyroxine.³³ In that study, a significantly higher concentration of BPA was found in cats fed with canned food and cats living indoors only. A higher concentration of BPA in indoor cats might be associated with house dust, where the presence of BPA has been demonstrated in previous studies.^59,60 Contaminated dust may be ingested during grooming, an important part of normal feline behaviour. BPA from house dust may be also rapidly absorbed through the oral mucosa via sublingual exposure.⁶¹ According to these results, it is likely that the environment in which the animals live is of similar importance to the food that is fed. Although it is difficult to decrease environmental exposure to BPA, the correct choice of food with lower levels of BPA may help. From the possible BPA exposure point of view, we can recommend offering the safer food from pouches or food trays, when feeding of wet food is necessary.

There were several limitations in this study: the small sample size of foods, the limited sources of pet food and the acquisition of samples in a single country only. Diet is considered the primary route of exposure to BPA and a previous report in dogs found a significant increase in serum BPA levels after feeding canned food. Further studies are needed to evaluate if the same consequence would occur in cats fed with canned food. Another question that has to be answered is the actual effect of BPA from food on feline metabolism and health.

Conclusions

The highest concentrations of BPA were found in cans; thus cans represent the highest possibility of exposure to BPA in comparison with other types of commercial cat food. On the other hand, there are significant differences between cans originating from individual pet food companies. The choice of pouches and food trays may decrease the exposure to BPA.

Supplemental Material

Supplementary Table 1

Characteristics of feline pet foods used in this study and BPA concentrations.

Supplemental Material

Supplementary Table 2

Values for extraction recovery, matrix effects, process efficiency, inter-day precision, accuracy, limits of detection (LOD) and lower limit of quantification (LLOQ).

Supplemental Material

Supplementary Figure 1

Chromatograms of a pouch sample and a can sample

Footnotes

Acknowledgements

The authors would like to thank Miss Karla Štěpánková and Miss Kristýna Trávníčková for their valuable help with the laboratory work.

Accepted: 6 April 2021

Conflict of interest

The authors declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The authors disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: this research was supported by the internal grant FVHE/Večerek/ITA2019 of the University of Veterinary and Pharmaceutical Sciences Brno.

Ethical approval

This work did not involve the use of animals and therefore ethical approval was not specifically required for publication in JFMS.

Informed consent

This work did not involve the use of animals and therefore informed consent was not required. No animals or humans are identifiable within this publication, and therefore additional informed consent for publication was not required.

ORCID iD

Simona Kovaříková

Supplementary material

The following files are available online:

Supplementary Table 1: Characteristics of feline pet foods used in this study and BPA concentrations.

Supplementary Table 2: Values for extraction recovery, matrix effects, process efficiency, inter-day precision, accuracy, limits of detection (LOD) and lower limit of quantification (LLOQ).

Supplementary Figure 1: Chromatograms of a pouch sample and a can sample.

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